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Reset Circuit

Posted: Sat Dec 12, 2020 8:41 am
by GianDO
Hi,
what is the component corresponding to the NE555 of the A500 on the A2000?

Thanks
G

Re: Reset Circuit

Posted: Sat Dec 12, 2020 10:23 am
by GadgetUK164
PST518B - It's connected up to an LM339. Either of those could cause reset problems.

I believe the 339 is used to reset based on the keyboard clock signal (ie. when you press Ctrl+Amiga+amiga) and perhaps has a "power good" type thing going on too where it can influence a reset?

It is confusing coming from the A500!

Edit: The above is for R6, the older boards use different components:-

LM2901 - U20
7438 - U16 (related to _HLT)
7407 - U12

Re: Reset Circuit

Posted: Sat Dec 12, 2020 12:56 pm
by GianDO
GadgetUK164 wrote:
Sat Dec 12, 2020 10:23 am
PST518B - It's connected up to an LM339. Either of those could cause reset problems.

I believe the 339 is used to reset based on the keyboard clock signal (ie. when you press Ctrl+Amiga+amiga) and perhaps has a "power good" type thing going on too where it can influence a reset?

It is confusing coming from the A500!

Edit: The above is for R6, the older boards use different components:-

LM2901 - U20
7438 - U16 (related to _HLT)
7407 - U12
I found a Rev6.2 which apparently has no damage, the battery seems to have been removed in time, but when turned on it gives a black screen. Somebody told me that the problem could be in the reset circuit and he thought it was the NE555 which is a chip that provides the reset signal to 68000 and start the A2000, but on the A2000 NE555 is not there, but it should correspond to the LM339.

So what I could do was remove all the socketed chip and try them on a working A2000 motherboard and luckily they all work, I tried them one by one: MC68000, Agnus, Gary, Kickstart, Denise, Paula, the two CIAs and the Buster. Putting them one by one on the working motherboard it turned on without problems. So at least these I would exclude them.
Then I did the test with the tester on pin 18 of 68000 and when I turned it on I immediately detected a peak of 4.95V which remained stable between 4.95 / 4.96V. It's normal?

Re: Reset Circuit

Posted: Sat Dec 12, 2020 8:46 pm
by GadgetUK164
Yes, the reset should go low on power on, then high as it comes out of reset.

Re: Reset Circuit

Posted: Sat Dec 12, 2020 11:24 pm
by GianDO
GadgetUK164 wrote:
Sat Dec 12, 2020 8:46 pm
Yes, the reset should go low on power on, then high as it comes out of reset.
As soon as I turn on the A2000 it is at 0V and after one second from switching on it goes to about 5V and remains like this ..

Re: Reset Circuit

Posted: Sat Dec 12, 2020 11:51 pm
by GadgetUK164
That's correct! It will go low for a second or so, then high. So it sounds like your reset is OK. It's probably corrosion related or a failed 245 on the buster side. Try running DiagROM!

Re: Reset Circuit

Posted: Sun Dec 13, 2020 8:52 am
by GianDO
GadgetUK164 wrote:
Sat Dec 12, 2020 11:51 pm
That's correct! It will go low for a second or so, then high. So it sounds like your reset is OK. It's probably corrosion related or a failed 245 on the buster side. Try running DiagROM!
Yes, I could try, although I believe DiagROM assumes that some components are functional in order to boot. I don't think it starts regardless ..
I'll try it.

Thanks
G

Re: Reset Circuit

Posted: Sun Dec 13, 2020 11:21 am
by GadgetUK164
If it doesn't start even though Diag ROM says everything is OK, it's probably going to be something Zorro related.